The first web services that gained traction were primarily based on XML. XML or eXtensible Markup Language was at the time the interchange format of choice in distributed computing and in the web environment. There were several different approaches to designing services with an XML payload.
It is possible that you may want to interact with existing XML-based web services that are developed either internally within your organization or externally. However, we advise you to implement new web services using more lightweight methods, such as JSON-based web services, RESTful web services, or gRPC.